A variable-director C-13 NMR analysis of lyotropic aramide solutions

The order and dynamics of two aromatic polyamides in their lyotropic phases were investigated with the aid of variable-director nuclear magnetic reson ance (NMR). In these experiments polymers were dissolved in concentrated su lfuric acid and allowed to equilibrate inside the main NMR magnetic field B -0 to yield macroscopically-aligned liquid crystalline solutions. These ord ered fluids were then rotated away from equilibrium for brief periods of ti me, and their natural abundance C-13 NMR spectra collected as a function of different angles between the liquid crystalline director and B-0. The resu lting spectra showed peaks shifting as well as broadening as a function of the director's orientation, variations that were also found to be concentra tion- and temperature-dependent. All such changes could be successfully acc ounted for on the basis of an exchange model involving molecular reorientat ions of the polymer chains that are occurring in the intermediate NMR time scale. Based on this assumption, the experimental line shapes could be used to extract a detailed description of the macromolecular order and dynamics in these fluids. The former appeared substantially high, and not very diff erent from the one characterizing order in commercial extruded aramide fibe rs. The latter enabled an estimation of the hydrodynamic radii adopted by t he macromolecules in their mesophases, which ended up in close agreement wi th dimensions recently reported on the basis of small-angle neutron scatter ing analyses. (C) 2001 American Institute of Physics.